1. Field of the Invention
The present invention relates to a process for forming a microstructure of a nitride semiconductor, a surface emitting laser using a two-dimensional photonic crystal and a production process thereof.
In particular, the present invention relates to a formation process for forming the microstructure inside the nitride semiconductor, and relates to a technology to be used for a process for producing a light-emitting device and the like using a photonic crystal.
2. Description of the Related Art
As for the surface emitting laser, a surface emitting laser is known which uses a two-dimensional photonic crystal for a reflector.
In a surface emitting laser using a nitride semiconductor that can emit light in a region from near ultraviolet to green, in particular, a distribution Bragg reflector which is generally used is hard to be produced, and accordingly, the surface emitting laser using a two-dimensional photonic crystal has been actively studied.
The photonic crystal has a microstructure in which refractive index is modulated with a period of the wavelength of light or less. Among the photonic crystals, a photonic crystal which functions in a visible region includes a plurality of holes with sizes of the order of several tens to one hundred and several tens nanometers.
When the photonic crystal is structured so as to be embedded inside a semiconductor, an advanced production technology is needed.
On the other hand, other semiconductor layers or electrodes can be laminated with semiconductor layers which sandwich the embedded photonic crystal, and accordingly, the photonic crystal has such an advantage that a photonic-crystal optical device in which an electric current can be injected in a lamination direction can be realized.
Japanese Patent Application Laid-Open No. 2004-111766 discloses a technology for forming holes inside a nitride semiconductor by using a mass transport phenomenon, and also a process for producing a surface emitting laser using the photonic crystal of the nitride semiconductor.
The specific technique is as follows.
Firstly, a hole is formed on the surface of a nitride semiconductor by EB lithography and dry etching.
An SiO2 hard mask is used in the dry etching process.
Next, after the above described hole has been formed, the above described hard mask is removed, and the nitride semiconductor is subjected to heat treatment at 1,000° C. under a gas atmosphere including nitrogen.
As a result, the mass transport of the surface atoms occurs, and eventually, the upper part of the above described hole is covered.
Then, a laser structure containing an active layer is epitaxially grown on the photonic crystal, and the surface emitting laser of the nitride semiconductor is produced.
In addition, a multilayer film of gallium nitride (GaN) and aluminum gallium nitride (AlGaN) in which the mass transport phenomenon is hard to occur as compared to the GaN is used.
Japanese Patent Application Laid-Open No. 2004-111766 adopts a technique for thereby enhancing the accuracy of the position in the depth direction along which the hole is formed.
The optical characteristics of the photonic crystal depend on the size and the shape of the hole.
In order to obtain a photonic crystal device having designed characteristics, the size and the shape of the hole need to be controlled with high accuracy.
In other words, if the size of the hole largely varies in the production process, it is difficult to obtain a photonic crystal having adequate optical characteristics.
A technology of etching a semiconductor in the etching process for forming a hole on the surface of the semiconductor is established, and the size of the hole and the variation within the plane of the hole can be controlled with high accuracy, in this production process.
However, the technique disclosed in Japanese Patent Application Laid-Open No. 2004-111766 has such a problem that after a heat treatment process which follows the etching process of forming the hole on the surface of the semiconductor, the size of the hole becomes larger than that of the hole before the heat treatment process.
In view of the above described problems, an object of the present invention is to provide a process for forming a microstructure of a nitride semiconductor, which can form the microstructure containing holes inside the semiconductor, without largely varying the size of the hole that has been formed by being precisely controlled in an etching process for the semiconductor, even after the semiconductor has been subjected to a heat treatment process; a surface emitting laser using a two-dimensional photonic crystal; and a production process thereof.